I read Castiel's reaction to Sam's awestruck OMG Angel's... as well Castiel being his Angel Wings Stuck up his Butt reaction, he was stating the truth while he shook Sam's hand, he didn't really have a opinion on Sam yet... He hadn't yet made a call... all he was saying was. So you are him, Sam, the one tainted by the demon blood. It was just a true statement about this man he was finally meeting.
Harsh yes, it came out wrong, of course, but it wasn't about putting Sam down. But of course it hurt Sam who was awestruck.
I do love how Castiel's arch in Season 6 was so much the same as Sam's in season 4... Pride and arrogance commeth before the the fall. They both thought they could handle this power, and they both couldn't... and they both screwed up royally in not turning to Dean sooner, when they should've and Dean screwed up by not being there/listening sooner... it was just.. sigh.
